EMPIRICAL MOLECULAR-DYNAMICS STUDY OF DIFFUSION IN LIQUID SEMICONDUCTORS

We report the results of an extensive molecular-dynamics study of diff usion in liquid Si and Ge (l-Si and l-Ge) and of impurities in l-Ge, u sing empirical Stillinger-Weber (SW) potentials with several choices o f parameters. We use a numerical algorithm in which the three-body par t of the SW potential is decomposed into products of two-body potentia ls, thereby permitting the study of large systems. One choice of SW pa rameters agrees very well with the observed l-Ge structure factors. Th e diffusion coefficients D(T) st melting are found to be approximately 6.4 x 10(-5) cm(2)/s for l-Si, in good agreement with previous calcul ations, and about 4.2 x 10(-5) and 4.6 x 10(-5) cm(2)/s for two models of l-Ge. In all cases, D(T) can be fitted to an activated temperature dependence, with activation energies E(d) of about 0.42 eV for l-Si, and 0.32 or 0.26 eV for two models of l-Ge, as calculated from either the Einstein relation or from a Green-Kubo-type integration of the vel ocity autocorrelation function. D(T) for Si impurities in l-Ge is foun d to be very similar to the selfdiffusion coefficient of l-Ge. We brie fly discuss possible reasons why the SW potentials give D(T)'s substan tially lower than ab initio predictions.